Narcolepsy
Medical Author: John P. Cunha, DO, FACOEP
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR
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What is narcolepsy?
Narcolepsy is a chronic disease of the central nervous system. Excessive daytime sleepiness (EDS) is the main symptom and is present in 100% of patients with narcolepsy. Other primary symptoms of narcolepsy include:
- loss of muscle tone (cataplexy),
- distorted perceptions (hypnagogic hallucinations), and
- inability to move or talk (sleep paralysis).
Additional symptoms include disturbed nocturnal sleep and automatic behavior (patients carry out certain actions without conscious awareness). All of the symptoms of narcolepsy may be present in various combinations and degrees of severity.
Narcolepsy usually begins in teenagers or young adults and affects both sexes equally. The first symptom to appear is
excessive daytime sleepiness, which may remain unrecognized for a long time in that it develops gradually over time. The other symptoms can follow
excessive daytime sleepiness by months or years.
How common is narcolepsy?
The prevalence of narcolepsy is similar to that of Parkinson's disease and multiple sclerosis. In the United States, the National Institute of Neurological Disorders and Stroke estimates narcolepsy affects one in every 2,000 people. However, in some countries (for
example, Israel), the prevalence of narcolepsy is much lower (one per 500,000) while in other countries (for
example, Japan), it is much higher (one per 600). The American Sleep Association estimates that approximately 125,000 to 200,000 Americans suffer from narcolepsy, but only fewer than 50,000 are properly diagnosed.
Narcolepsy often remains undiagnosed or misdiagnosed for several years. This may occur because physicians do not consider the diagnosis of narcolepsy frequently enough. They may think of narcolepsy only in people who have the main symptom of
excessive daytime sleepiness. Narcolepsy may not be considered in the evaluation of patients who come to doctors complaining of fatigue, tiredness, or problems with concentration, attention, memory, and performance, and other illnesses (seizures, mental illness, etc.).
What causes narcolepsy?
Advances have been made in the last few years in determining the cause of narcolepsy. The newest discovery has been the finding of abnormalities in the structure and function of a particular group of nerve cells, called hypocretin neurons, in the brains of patients with narcolepsy. These cells are located in a part of the brain called the hypothalamus and they normally secrete neurotransmitter substances (chemicals released by nerve cells to transmit messages to other cells) called hypocretins.
Abnormalities in the hypocretin system may be responsible for the daytime sleepiness and abnormal REM sleep found in narcolepsy. (See the section below on sleep laboratory tests for a discussion of
REM sleep.)
Experiments in dogs and mice with narcolepsy point to an abnormal hypocretin system as a cause for the development of their narcolepsy. People with narcolepsy have been found to have a markedly decreased number of hypocretin nerve cells in the brain. They also have a decreased level of hypocretins in the cerebrospinal fluid (the fluid that surrounds the brain and the spinal cord).
Narcolepsy is associated with a specific type of human leukocyte antigen (HLA). HLAs are genetically determined proteins on the surface of
white blood cells. They are a part of the body's immune (defense) system. The finding of a very high HLA-association in narcolepsy led to the proposal that narcolepsy is an
autoimmune disease, similar to other HLA-associated diseases such as multiple sclerosis and ankylosing spondylitis.
It is theorized that an autoimmune reaction causes the loss of nerve cells in the brain in patients with narcolepsy. The environment (for example, infection or trauma) might trigger an autoimmune reaction where normal brain cells are attacked by the body's own immune system. As a result, the neurons are damaged and ultimately destroyed, and they and their neurotransmitter chemicals disappear. Whether narcolepsy is an autoimmune disease remains to be proven.
The role of heredity in humans with narcolepsy is not completely understood. No consistent pattern of heredity has been recognized in families so far. It is estimated that relatives of patients with narcolepsy may have a higher predisposition to develop narcolepsy or sleep-related abnormalities, such as increased daytime sleepiness, increased REM sleep, or others. In dogs with narcolepsy, the disease is inherited in a predictable pattern. In these animals, the narcolepsy is caused by a mutation in a particular gene that is normally responsible for producing a receptor (binder) in the brain for the hypocretin neurotransmitter.
Next: What are the symptoms of narcolepsy? »
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